Consolidated Statements of Stockholders' Equity/Deficit-Unaudited - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Gain (Loss)	Treasury Stock
Balance, shares at Dec. 31, 2019		106,679
Balance, value at Dec. 31, 2019	$ 117,101	$ 106	$ 1,462,172	$ (1,341,444)	$ 84	$ (3,817)
Stock-based compensation	4,432	0	4,432	0	0	0
Repurchase of common stock	(923)	0	0	0	0	(923)
Net loss		0	0	(66,611)	0	0
Unrealized gain on investments	776	$ 0	0	0	776	0
Balance, shares at Mar. 31, 2020		107,711
Balance, value at Mar. 31, 2020	54,777	$ 108	1,466,604	(1,408,055)	860	(4,740)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures		1,032
Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture	2	$ 2	0	0	0	0
Balance, shares at Dec. 31, 2019		106,679
Balance, value at Dec. 31, 2019	117,101	$ 106	1,462,172	(1,341,444)	84	(3,817)
Net loss	(53,079)
Unrealized gain on investments	(49)
Balance, shares at Sep. 30, 2020		117,231
Balance, value at Sep. 30, 2020	87,639	$ 117	1,486,853	(1,394,523)	35	(4,843)
Balance, shares at Mar. 31, 2020		107,711
Balance, value at Mar. 31, 2020	54,777	$ 108	1,466,604	(1,408,055)	860	(4,740)
Stock-based compensation	4,258	$ 0	4,258	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		187
Issuance of common stock under Equity Incentive Plans, value	0	$ 0	0	0	0	0
Repurchase of common stock	(103)	0	0	0	0	(103)
Net loss		0	0	(69,071)	0	0
Unrealized gain on investments	(548)	$ 0	0	0	(548)	0
Balance, shares at Jun. 30, 2020		107,898
Balance, value at Jun. 30, 2020	(10,687)	$ 108	1,470,862	(1,477,126)	312	(4,843)
Stock-based compensation	1,904	$ 0	1,904	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		9,333
Issuance of common stock under Equity Incentive Plans, value	14,096	$ 9	14,087	0	0	0
Net loss	82,603	0	0	82,603	0	0
Unrealized gain on investments	(277)	$ 0	0	0	(277)	0
Balance, shares at Sep. 30, 2020		117,231
Balance, value at Sep. 30, 2020	$ 87,639	$ 117	1,486,853	(1,394,523)	35	(4,843)
Balance, shares at Dec. 31, 2020	142,289	142,289
Balance, value at Dec. 31, 2020	$ 156,371	$ 142	1,561,096	(1,400,018)	(6)	(4,843)
Stock-based compensation	2,851	$ 0	2,851	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		2,000
Issuance of common stock under Equity Incentive Plans, value	16,399	$ 2	16,397	0	0	0
Repurchase of common stock	(2,675)	0	0	0	0	(2,675)
Net loss	(20,958)	0	0	(20,958)	0	0
Unrealized gain on investments	11	$ 0	0	0	11	0
Balance, shares at Mar. 31, 2021		145,552
Balance, value at Mar. 31, 2021	152,547	$ 145	1,580,891	(1,420,976)	5	(7,518)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures		1,263
Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture	$ 548	$ 1	547	0	0	0
Balance, shares at Dec. 31, 2020	142,289	142,289
Balance, value at Dec. 31, 2020	$ 156,371	$ 142	1,561,096	(1,400,018)	(6)	(4,843)
Net loss	(62,147)
Unrealized gain on investments	$ 1
Balance, shares at Sep. 30, 2021	149,987	149,987
Balance, value at Sep. 30, 2021	$ 136,603	$ 150	1,606,142	(1,462,165)	(6)	(7,518)
Balance, shares at Mar. 31, 2021		145,552
Balance, value at Mar. 31, 2021	152,547	$ 145	1,580,891	(1,420,976)	5	(7,518)
Stock-based compensation	$ 2,786	0	2,786	0	0	0
Issuance of common stock under Equity Incentive Plans, shares	88
Issuance of common stock under Equity Incentive Plans, value	$ 8	1	7	0	0	0
Payments of Stock Issuance Costs	31	0	(31)	0	0	0
Net loss	(18,067)	0	0	(18,067)	0	0
Unrealized gain on investments	(10)	$ 0	0	0	(10)	0
Balance, shares at Jun. 30, 2021		145,640
Balance, value at Jun. 30, 2021	137,233	$ 146	1,583,653	(1,439,043)	(5)	(7,518)
Stock-based compensation	$ 2,712	0	2,712	0	0	0
Issuance of common stock under Equity Incentive Plans, shares	4,177
Issuance of common stock under Equity Incentive Plans, value	$ 19,123	4	19,119	0	0	0
Net loss	(23,122)	0	0	(23,122)	0	0
OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax					(1)
Unrealized gain on investments	$ 0	$ 0	0	0		0
Balance, shares at Sep. 30, 2021	149,987	149,987
Balance, value at Sep. 30, 2021	$ 136,603	$ 150	1,606,142	(1,462,165)	(6)	(7,518)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures		170
Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture	$ 658	$ 0	$ 658	$ 0	$ 0	$ 0